Stainless clad steel with excellent corrosion resistance

ABSTRACT

A stainless clad steel includes a stainless steel having Pitting Index of 35 or more as a cladding material, wherein the ratio of Cr concentration/Fe concentration in a passivation film portion of the above-described cladding material to Cr concentration/Fe concentration in a parent phase portion of the above-described cladding material is 1.20 or more and, in addition, the amount of precipitation of a σ (sigma) phase of the surface of the above-described cladding material is 2.0% or less on an area ratio basis.

TECHNICAL FIELD

This disclosure relates to a stainless clad steel with excellent corrosion resistance, used for various application purposes typified by offshore structures, heat exchangers, chemical tankers, chemical plants, and pressure vessels.

BACKGROUND ART

In recent years, there has been a tendency of the temperature and pressure of a plant operation to increase from the viewpoint of improvement in efficiency. In the design of a chemical plant, the proportion of use of a steel sheet having a higher sheet thickness tends to increase to ensure strength. In addition, needs of industrial facilities and structures are directed toward durability, an increase in life, and freedom from maintenance, and stainless steel has become a focus of attention as a material conforming to these needs. Meanwhile, as for alloy elements typified by Ni, Mo, and Cr, which are main raw materials for the stainless steel, there are increases and fluctuations in prices. Consequently, instead of stainless steel, a stainless clad steel has been noted as a steel product recently, wherein excellent rusting resistance of the stainless steel can be utilized more economically, and the price is stable and low.

Stainless clad steel refers to a steel product in which two types of metals having different properties are bonded together, where a cladding material is a stainless steel and a base material is a carbon steel. The clad steel is produced by metallurgically bonding different types of metals so that, in contrast to coating, there is no fear of peeling and new characteristics, which are not exhibited by a single metal or an alloy, can be provided.

As for the stainless clad steel, to ensure rusting resistance which is a function serving the purpose on a use environment basis, the type of stainless steel serving as a cladding material is selected on a use environment basis and, thereby, the corrosion resistance equivalent to that in the case where the stainless steel is employed throughout the thickness (hereafter may be referred to as “solid material”).

As described above, stainless clad steel has advantages that compatibility between economy and functionality can be ensured because a usage of the stainless steel is reduced and the rusting resistance equivalent to that of a solid material can be ensured. Consequently, it is believed that stainless clad steel is a very useful functional steel product, and needs for the stainless clad steel have increased in various industrial fields.

In particular, in many cases, stainless clad steel is used for application purposes requiring corrosion resistance. Therefore, enhancement in functionality of the surface is an important technical issue. In this regard, in the prior art, a technique in which the corrosion resistance conforming to the requirement is ensured by selecting a stainless steel used as a cladding material is employed in general to ensure the corrosion resistance of the stainless clad steel sheet required on an application purpose (for example, offshore structures, heat exchangers, chemical plants, chemical tankers, and pressure vessels, etc.) basis. However, in stainless clad steel, it is difficult to ensure compatibility between improvement in the soundness and reliability of the bonding interface and performances of the base material and the cladding material with respect to all high grade steel products and the variety of types.

It is not said that the above-described technology to improve the function, e.g., corrosion resistance, by appropriately controlling the surface quality has been studied sufficiently in the same component system.

Examples of technologies to improve the corrosion resistance of the stainless clad steel include Japanese Patent No. 4179133, Japanese Patent No. 3409660, Japanese Patent No. 3514889 and Japanese Patent No. 3401538.

Japanese Patent No. 4179133 discloses a method in which, in a method of manufacturing a stainless clad steel pipe by employing a stainless steel with excellent corrosion resistance in sea water as a cladding material and carbon steel as a base material, the solid solution heat treatment condition is specified and the components of the base material carbon steel are specified to be within appropriate component ranges to recover degradation in the corrosion resistance of a seam weld zone. However, that method prevents precipitation of a second phase that degrades the corrosion resistance, and study on the surface quality of the stainless steel is not performed. That is, the surface quality, e.g., a Cr concentration ratio in a passivation film and the glossiness of the surface, has not been studied. Therefore, a dramatic improvement in the corrosion resistance is not expected.

Japanese Patent No. 3409660 discloses a thin stainless clad steel sheet by employing an austenite stainless steel as a cladding material and a low-carbon steel as a base material among thin stainless clad steel sheets suitable for raw materials to be subjected to working, e.g., deep drawing, punch stretching, and bending. It is disclosed that the low-carbon steel serving as the base material contains C: 0.015 to 0.06 percent by weight, N: 0.010 percent by weight or less, Ti: 0.10 to 0.40 percent by weight, and B: 0.0005 to 0.0050 percent by weight and has a component composition satisfying (Ti−3.4N)/4C≧0.6 and Ti×C=2.8×(1/103) to 13.5×(1/103) (where Ti: Ti content (percent by weight), N: N content (percent by weight), and C: C content (percent by weight)) and, thereby, even when high-temperature annealing is performed to subject to high deformation, base material crystal grains do not become coarse, surface roughness, e.g., orange peel, does not occur, and excellent surface quality is exhibited. However, Japanese Patent No. 3409660 does not disclose improvement of corrosion resistance, although a technology to reduce uneven shapes of the steel sheet surface in working is disclosed.

Japanese Patent No. 3514889 discloses an austenite stainless clad steel sheet used in fields, e.g., line pipes used in a sour gas environment, tanks in chemical tankers, and absorption containers for flue gas desulfurization apparatuses, in which high corrosion resistance is required and a manufacturing method, e.g., a heat treatment condition, thereof.

However, Japanese Patent No. 3514889 only discloses ensuring the steel sheet corrosion resistance basically by specifying the types of alloy components and adjusting the content, and a relationship with the technology related to the surface quality has not been studied.

Japanese Patent No. 3401538 discloses a super stainless/stainless clad steel sheet formed from a composite metal sheet with excellent corrosion resistance and formability. In that clad steel sheet, a super stainless steel containing Ni, Cr, Mo, and N and having a composition satisfying the condition of (Cr+2×Mo+9×N)≧27% (percent by weight), 16%≦Ni≦30%, 18%≦Cr≦30%, 7%<Mo≦8%, and 0.10%≦N is employed as a cladding material on both surfaces or one surface of the stainless steel and the interface between the super stainless steel and the stainless steel is metallurgically bonded. However, in Japanese Patent No. 3401538 as well, corrosion resistance is enhanced by alloy elements and, therefore, it is difficult to improve the characteristics without increasing the amount of addition of the alloy elements.

As described above, almost all technologies improve the corrosion resistance of the stainless clad steel by heat treatment methods or adjustment of alloy elements and improving the corrosion resistance by controlling the surface quality of a final product has not been studied sufficiently.

It could therefore be helpful to provide a stainless clad steel excellent in corrosion resistance, in particular, rusting resistance and an improvement in appearance due to prevention of discoloration and which can prevent an occurrence of outflow rust.

SUMMARY

We subjected stainless clad steel sheets by using stainless steels which had the same components (steel composition), which had the same history of completion of rolling to a heat treatment, and which had Pitting Index of 35 or more to various mirror finish treatments and performed studies on the surface quality in detail. The Pitting Index is represented by (Cr+3.3Mo+16N), where each symbol of element indicates the content on a percent by mass basis of the element concerned. Stainless steel is a generic name for the steel product which is a Fe—Cr or Fe—Cr—Ni alloy steel having the corrosion resistance in such a way as to become applicable to uses in need of corrosion resistance in sea water and which is characterized by containing Cr at a high content (for example, 20% or more) and Mo (for example, 2% or more), wherein a σ phase (intermetallic compound) is precipitated in a manufacturing process.

We noted the surface glossiness and anisotropy thereof, the strength of passivation film having an influence on the corrosion resistance, especially pitting resistance of the stainless steel, the Cr/Fe ratio, and the like and performed studies. As a result, we found that the rusting resistance was improved considerably by specifying the ratio of Cr concentration (atomic percent)/Fe concentration (atomic percent) in the passivation film portion to Cr concentration (atomic percent)/Fe concentration (atomic percent) in the stainless steel serving as a parent phase portion to be 1.20 or more and an average glossiness indicator of 60-degree specular glossiness; Gs(60°) in a rolling direction (L), that in a perpendicular direction (C), and that in a direction at an angle of 45 degrees with respect to the rolling direction (D) measured by JIS Z 8741 (1997) “Specular glossiness-methods of measurement” of the above-described stainless clad steel surface to be 60 or more.

We thus provide:

-   [1] A stainless clad steel characterized by including a stainless     steel having Pitting Index represented by Formula (1) described     below of 35 or more as a cladding material, -   wherein the ratio of Cr concentration (atomic percent)/Fe     concentration (atomic percent) in a passivation film portion of the     above-described cladding material to Cr concentration (atomic     percent)/Fe concentration (atomic percent) in a parent phase portion     of the above-described cladding material is 1.2 or more and, in     addition, the amount of precipitation of a σ (sigma) phase of the     surface is 2.0% or less on an area ratio basis,

Pitting Index=(Cr+3.3Mo+16N)   Formula (1)

Where Cr, Mo, and N are on a percent by mass basis, and the case of no inclusion is specified to be 0.

-   [2] The stainless clad steel according to the item [1],     characterized in that the average glossiness indicator Gs(60)     calculated by Formula (2) described below of the above-described     cladding material is 60 or more, where the specular glossiness at a     measurement angle of 60° defined in JIS Z 8741 is measured in a     rolling direction (Gs(60)_(L)), in a direction perpendicular to the     rolling direction (Gs(60)_(C)), and in a direction at an angle of 45     degrees with respect to the rolling direction (Gs(60)_(D)),

Gs(60)=(Gs(60)_(L)+2×Gs(60)_(D)+Gs(60)_(C))/4   Formula (2).

-   [3] The stainless clad steel according to the item [1] or [2],     characterized in that the average glossiness indicators Gs(60)_(L),     Gs(60)_(C), Gs(60)_(D) and Gs(60) are 60 or more, where the specular     glossiness at a measurement angle of 60° defined in JIS Z 8741.

A stainless clad steel with improved corrosion resistance, in particular, rusting resistance can be provided, wherein an appearance is improved because of prevention of discoloration of the surface and outflow rust can be improved.

In particular, a stainless clad steel with excellent appearance due to corrosion resistance, especially prevention of outflow rust, is favorably used for various application purposes typified by offshore structures, heat exchangers, chemical tankers, chemical plants, and pressure vessels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing measurement examples of the Cr concentrations (atomic percent) and the Fe concentrations (atomic percent) of a passivation film and a stainless steel parent phase portion.

FIG. 2 is a diagram showing the measurement condition of the surface glossiness.

DETAILED DESCRIPTION

The configuration of our steel sheets and stainless clad steel will be described.

(1) Initially, the stainless clad steel is a stainless clad steel by using a stainless steel having Pitting Index represented by Formula (1) of 35 or more as a cladding material. Pitting Index=(Cr+3.3Mo+16N) . . . (1) Cr, Mo, and N indicate the contents (percent by mass) of their respective elements, and the case of no inclusion is specified to be 0.

In this regard, as for the stainless steel serving as the cladding material, Pitting Index (Cr+3.3Mo+16N)≧35 is selected because when uses in need of the corrosion resistance (in particular, crevice corrosion resistance) are considered among various application purposes typified by offshore structures, heat exchangers, chemical tankers, chemical plants, and pressure vessels, it is believed to be necessary to contain much Cr and Mo effective in preventing occurrence and growth of pitting and have Pitting Index serving as an indicator of pitting resistance of 35 or more. In this regard, the relationship between Pitting Index and CPT (Critical Pitting Temperature (ASTM G48-03 Method E)) or CCT (Critical Crevice Temperature (ASTM G48-03 Method D)) is a positive interrelation.

(2) Next, the amount of precipitation of a σ (sigma) phase of the surface of the cladding material is 2.0% or less on an area ratio basis.

As described above, a high-alloy stainless clad steel which can improve an appearance because of the corrosion resistance, in particular, rusting resistance, and prevention of discoloration of the surface and which can improve outflow rust is obtained by controlling the surface quality and, in addition, specifying precipitation of the σ phase in the steel to be 2.0% or less.

We ascertained that a σ phase precipitated in the steel containing Mo, was an intermetallic compound containing FeCrMo by identifying a residue extracted by a Speed method in a 10% acetylacetone-1% tetramethylammonium mixed electrolytic solution (commonly called AA solution) on the basis of X-ray diffraction.

If the σ phase is precipitated, Cr and Mo in the steel are reduced. It is preferable that the amount of precipitation of σ phase be minimized because Cr and Mo in the steel are alloy elements effective in improving the corrosion resistance.

Meanwhile, in many cases, a heat treatment (Q-T treatment) is performed in consideration of a production process of a clad steel and strength-toughness of a base material. In that case, precipitation of a σ phase in the heat treatment and a cooling process thereof is not avoided depending on the heat treatment and cooling conditions. In addition, in hot rolling and a cooling process thereof as well, precipitation of a σ phase may occur to cause significant degradation in corrosion resistance.

The relationship between the amount of precipitation of the σ phase and the corrosion resistance of the stainless steel (cladding material) surface was examined. As a result, we found that when the amount of precipitation of the σ phase was more than 2.0%, the corrosion resistance was degraded significantly and an occurrence of rusting of the surface was conspicuous. The reason for this is believed to be that if the amount of precipitation of the σ phase is more than 2.0%, σ phases precipitated at grain boundaries are joined and cover the grain boundaries and, thereby, corrosion is conspicuous.

(3) The ratio of Cr concentration (atomic percent)/Fe concentration (atomic percent) in a passivation film portion of the cladding material to Cr concentration (atomic percent)/Fe concentration (atomic percent) of a parent phase portion of the above-described cladding material is 1.20 or more.

The Cr concentration (atomic percent)/Fe concentration (atomic percent) in the passivation film portion of the cladding material is very important factor in an improvement of pitting resistance. As the ratio (I₁/I₂, hereafter abbreviated as Cr/Fe concentration ratio) of Cr concentration (atomic percent)/Fe concentration (atomic percent); I₁ in the passivation film portion of the stainless steel serving as the cladding material to Cr concentration (atomic percent)/Fe concentration (atomic percent); I₂ in a parent phase portion becomes high, a stable passivation film with excellent pitting resistance is disposed as the surface layer. Therefore, a higher Cr/Fe concentration ratio is better from the viewpoint of the corrosion resistance. We found that the Cr/Fe concentration ratio of 1.20 or more was necessary to exert a corrosion resistance (pitting resistance) improving effect clearly on the basis of an atmospheric corrosion test or an accelerated corrosion test as compared with an abrasive. The Cr/Fe concentration ratio is specified to be 1.20 or more on the basis of this finding. Preferably, the Cr/Fe concentration ratio is 1.50 or more.

On the other hand, to increase the Cr/Fe concentration ratio to a great extent, acid dipping, pickling, or an electrolytic treatment is necessary. The stainless clad steel sheet is a clad steel sheet of a carbon steel and a stainless steel. Therefore, in a dipping treatment in a predetermined solution, it is necessary to make considerations in such a way that the common steel is not dissolved, and an application of a load is necessary to improve the degree of concentration of Cr in the surface layer to higher than or equal to that of a solid material (stainless steel). As described above, an equipment load is required for an excessive improvement in the Cr/Fe concentration ratio. Therefore, preferable upper limit is 5.0 or less.

In this regard, an improvement in the Cr/Fe concentration ratio is not expected by only mechanical polishing, so that it is important to combine with some chemical surface control technique. The surface-controlling technique can be a combination of various known surface-polishing techniques, e.g., common belt polishing, grinder polishing, abrasive wheel polishing, electrolytic polishing, and a pickling treatment. The surface roughness and the anisotropy thereof are controlled at low levels by combining the above-described techniques and, in addition, the passivation film is strengthened, so that desired characteristics are obtained. Examples of methods of strengthening the passivation film of the surface include a technique by using a pickling treatment with nitric acid or fluoro-nitric acid or electrolytic polishing. It is also possible to combine these methods with an electrolytic neutral pickling treatment in a neutral salt solution (for example, Ruther process: 20% sodium sulfate solution and sodium nitrate).

Meanwhile, as for the Cr/Fe concentration ratio, the Cr/Fe concentration ratio can be determined by, for example, measuring the concentration profile (atomic percent) of an element while the steel surface is sputtered in the depth direction and determining an atomic ratio of Cr to Fe from the individual element (Fe, Cr, and the like) concentration profiles. In this case, as shown in FIG. 1, a region in which the values (atomic percent) of Cr and Fe are almost constant values is assumed to be a parent phase portion, and a region with a sputtering time shorter than that is defined as a passivation film portion. As for the passivation film portion, the value at the site exhibiting a highest Cr/Fe value is defined as the Cr concentration (atomic percent)/Fe concentration (atomic percent) in the passivation film portion.

(4) Next, it is preferable that the average glossiness indicator Gs(60) calculated by Formula (2) be 60 or more, where the specular glossiness at a measurement angle of 60° defined in JIS Z 8741 (1997) (Specular glossiness-methods of measurement) is measured in a rolling direction (Gs(60)_(L)), in a direction perpendicular to the rolling direction (Gs(60)_(C)), and in a direction at an angle of 45 degrees with respect to the rolling direction (Gs(60)_(D)).

Gs(60)=(Gs(60)_(L)+2×Gs(60)_(D)+Gs(60)_(C))/4   (2)

It is more preferable that the average glossiness indicators Gs(60)_(L), Gs(60)_(C), Gs(60)_(D) and Gs(60) are 60 or more, where the specular glossiness at a measurement angle of 60° defined in JIS Z 8741.

The surface glossiness is an indicator showing fine unevenness of the surface and is an evaluation indicator of the surface quality. Fine surface roughness evaluated by the surface glossiness influences not only the indicator of appearance of the stainless clad steel, but also degradation in pitting resistance and the form of outflow rust to a great extent.

We believe that the metal glossiness well reflected the surface roughness, that is, characteristics of micro-unevenness of the surface and the surface roughness was noted. As for a surface material with high glossiness and less anisotropy, soiling does not occur easily, an occurrence of crevice corrosion is prevented, and the form of rust tends to become spot rust rather than outflow rust because of less anisotropy. In addition, even when rusting occurs once, there is a high possibility of being washed away by rainwater. Furthermore, there is an advantage that maintenance is easy because fine unevenness is reduced and the surface is smooth.

As described above, we believe that surface fine unevenness has a large influence on the rusting resistance of the stainless clad steel. For example, in use in seawater, as unevenness increases, seawater, marine microbes, and contaminants in the ocean adhere to the surface easily. Consequently, micro gaps are formed resulting from them, and crevice corrosion occurs easily.

Moreover, when the glossiness has anisotropy, solution components tend to remain in a specific direction, so that discoloration and rusting are induced especially at a gas-liquid interface. Also, color tones are different and, therefore, there is a limit in practical use, that is, when the clad steel is used, it is necessary to apply in consideration of, for example, the direction of taking of the sheet.

Then, we performed on the relationship between the surface glossiness and the corrosion resistance on the basis of those described above. As a result, we found that the corrosion resistance was improved when the average glossiness indicator (Gs(60)) calculated by Formula (2) was 60 or more, where the glossiness (Gs(60)) defined in JIS Z 8741 “Specular glossiness-methods of measurement” was measured in a rolling direction (L), in a perpendicular direction (C), and in a direction at an angle of 45 degrees with respect to the rolling direction (D).

Preferably, the glossiness (Gs(60)) is higher. However, polishing of the stainless steel to increase the glossiness requires a significant load. Consequently, in consideration of the production load, the upper value of Gs(60) measured in the individual directions of the rolling direction (L), the perpendicular direction (C), and the direction at an angle of 45 degrees with respect to the rolling direction (D) is 300, and preferably 150 or less.

Meanwhile, as for each glossiness and the average glossiness indicator, the surface glossiness can be measured on the basis of JIS Z 8741 “Specular glossiness-methods of measurement” by using a multi-angle gloss meter under the condition shown in FIG. 2. Gs(60) can be derived by an average of five points.

(5) In this connection, as for the stainless clad steel, any of a hot-rolled steel sheet and a steel sheet subjected to an annealing heat treatment after a hot-rolling treatment is included, and the same effect is obtained.

Also, the surface quality is controlled and the surface characteristics are allowed to fall within predetermined ranges by combining chemical treatments in addition to mechanical polishing. That is, surface-controlling technique can be a combination of various known surface-polishing techniques, e.g., common belt polishing, grinder polishing, abrasive wheel polishing, electrolytic polishing, and a pickling treatment. The surface roughness and the anisotropy thereof are controlled at low levels by combining the above-described techniques and, in addition, the passivation film is strengthened so that desired characteristics are obtained. Examples of methods of strengthening the passivation film of the surface include a technique by using a pickling treatment with nitric acid or fluoro-nitric acid or electrolytic polishing. It is also possible to combine these methods with an electrolytic neutral pickling treatment in a neutral salt solution (for example, Ruther process: 20% sodium sulfate solution and sodium nitrate).

(6) Meanwhile, a carbon steel and a low-alloy steel can be used as the base material of the stainless clad steel. Then, in the stainless clad steel, one surface or both surfaces of this base material is clad with stainless steel serving as a cladding material, and the method of cladding the base material with the cladding material is not specifically limited. A hot-rolling method, an explosive rolling method, a diffusion bonding method, a cast-in insert method, and the like can be used.

(7) Also, an annealing treatment to hold at a temperature of 700° C. to 1,000° C. for 1 minute to 2 hours can also be performed. When Cr and Mo contents contained in stainless steel used as a cladding material of a stainless clad steel are large, for example, in the case of high-alloy steel having a Cr content of 20% or more and containing 2% or more of Mo, a σ (sigma) phase, a χ (chi) phase, and furthermore, M₂₃C₆, M₆C (primary components of M are Fe, Cr), and the like are generated, so that effective Cr may be reduced and considerable degradation in corrosion resistance may be caused by sensitization. In such a case, the stainless clad steel with a controlled surface, is effective and can contribute to removal of a Cr-removing layer and putting of a sensitized portion into a sound basis.

EXAMPLES

Our steel sheets and stainless clad steel will be described below in detail. Each of molten steels of two types of high-alloy stainless steel having chemical compositions shown in Table 1 and exhibiting Pitting Index (Cr+3.3Mo+16N)≧35 and a common structural steel (equivalent to EH36) was refined by known methods, e.g., a converter, an electric furnace, and a vacuum melting furnace, and was made into a steel (slab) by a continuous casting method or an ingot making-slabbing method. Subsequently, the resulting steel was treated sequentially through hot rolling, hot-rolled sheet annealing (for example, box annealing), and pickling, so as to produce a hot-rolled sheet. In addition, cold rolling and finish annealing (for example, continuous annealing) were performed, so as to produce a cold-rolled annealed sheet. The resulting cold-rolled annealed sheet was used as a cladding material (austenite stainless steel) and a base material of a clad and a stainless clad steel was produced under the production conditions shown in Table 2.

That is, the cladding material (austenite stainless steel, sheet thickness 20 mm) and the base material (common structural steel: steel equivalent to EH36) shown in Table 1 were made into an assembled slab dimension 1,890 mm wide by 2,060 mm long, and a stainless clad steel (cladding material: sheet thickness 4.0 mm, base material: sheet thickness 14.0 mm, width 2,500 mm, length 8,000 mm) was produced under the condition of slab heating temperature (° C.): 1,150° C. to 1,250° C., finish rolling temperature (° C.): 1,000° C.±50° C., water cooling start temperature (° C.): 950° C.±50° C., water cooling finish temperature (° C.): 650° C.±50° C., and cooling rate (° C./s): 0.2° C./s to 7.0° C./s. In addition, part of the stainless clad steel was subjected to an annealing heat treatment at 910° C.±20° C. for 10 minutes or 2 hours, as shown in Table 3.

TABLE 1 (percent by mass) Pitting Cladding material C Si Mn P S Cr Ni Mo Cu N Index Remarks Cladding material 1 0.016 0.23 0.53 0.026 0.002 24.1 23.1 4.22 0.05 0.18 40.90 Austenitic Stainless Steel (SUS) Cladding material 2 0.015 0.41 0.61 0.021 0.004 20.3 18.5 6.62 0.75 0.20 45.35 Austenitic Stainless Steel (SUS) Base material 0.12 0.35 1.55 0.013 0.003 0.02 0.37 0.001 0.31 0.09 — EH36 (mild steel)

TABLE 2 Water cooling Water cooling Slab heating Finish rolling start finish Cooling Assembled slab temperature temperature temperature temperature rate Product dimension dimension (mm) (° C.) (° C.) (° C.) (° C.) (° C./s) (mm) Stainless steel 1150~1250° C. 1000 ± 50 950 ± 50 650 ± 50 0.2~7.0 Stainless steel (cladding material) (cladding material) thickness: 20 thickness: 4.0 Steel (base material) Steel (base material) thickness: 73 thickness: 14.0 width: 1890 width: 2500 length: 2060 length: 8000

TABLE 3 Production Cooling rate Annealing heat condition (° C./s) treatment condition Conition 1 0.3 none Conition 2 1.0 none Conition 3 7.0 none Conition 4 0.3 910 ± 20° C.-10 min Conition 5 0.3 910 ± 20° C.-120 min

The stainless clad steel obtained as described above was subjected to a combination of various known surface polishing techniques, e.g., common belt polishing, grinder polishing, abrasive wheel polishing, electrolytic polishing, and a pickling treatment so that the surface roughness and the anisotropy thereof were controlled at low levels. Furthermore, in addition to them, a pickling treatment with nitric acid, fluoro-nitric acid, or sulfuric acid or electrolytic polishing was performed for the purpose of strengthening the passivation film of the surface, so that desired characteristics were obtained.

The steel sheet with controlled surface quality was subjected to measurements of the Cr/Fe concentration ratio, the glossiness, and the pitting potential, a CCT test was performed, and the corrosion resistance was evaluated. The results obtained as described above are shown in Tables 4 and 5. Underlined values on Surface layer σ phase area ratio column and on Cr/Fe concentration ratio in passive film portion and on parent phase portion column show Comparative examples of claim 1, Underlined values on Average Gs(60) column show Comparative examples of claim 2. On Surface treatment cost column, ‘X’ indicates Surface treatment cost is more than 2.0 times, ‘Δ’ indicates Surface treatment cost is more than 1.5 times and 2.0 times or less, and ‘◯’ indicates Surface treatment cost is less than 1.5 times, compared to Steel No. 1 of Table 4.

TABLE 4 Test results Cr/Fe concentration ratio in passive film Pitting portion and parent Surface potential Clad- Aver- phase portion layer σ measurement CPT test ding age (relative value phase 70° C. evaluation(A Surface mate- Production Gs Gs Gs Gs based on AES area ratio (V′C100 mV STM G48-03 treatment No. rial condition (60)_(L) (60)_(D) (60)_(C) (60) analysis)* (%) vs SCE) Method E) cost Remarks 1 1 Condition 3 59.3 48.6 29.6 46.5 0.99 <1.0 100 X ◯ Comparative example 2 1 Condition 3 88.3 50.3 39.1 57.0 1.05 <1.0 109 X ◯ Comparative example 3 1 Condition 3 70.0 50.3 55.0 56.4 1.80 <1.0 305 ◯ ◯ example 4 1 Condition 3 63.4 59.0 68.3 62.4 1.44 ≦2.0  300 ◯ ◯ example 5 1 Condition 3 19.8 16.3 28.0 20.1 0.99 <1.0 115 X ◯ Comparative example 6 1 Condition 3 88.2 96.0 88.1 92.1 0.99 <1.0 133 X ◯ Comparative example 7 1 Condition 3 169.0 128.4 133.4 139.8  1.03 <1.0 240 X ◯ Comparative example 8 1 Condition 3 150.3 100.9 88.1 110.1  2.01 <1.0 489 ◯ ◯ example 9 1 Condition 3 156.0 184.1 122.9 161.8  6.03 <1.0 690 ◯ Δ example 10 1 Condition 3 220.1 155.3 180.0 177.7  1.35 <1.0 306 ◯ Δ example 11 1 Condition 3 251.0 200.3 221.0 218.2  1.45 <1.0 336 ◯ X example 12 1 Condition 3 251.0 200.3 221.0 218.2  0.98 <1.0 183 X X Comparative example 13 1 Condition 1 120.0 55.0 111.3 85.3 1.21 0  305 ◯ ◯ example 14 1 Condition 1 140.3 59.0 111.3 92.4 1.10  3.2 180 X ◯ Comparative example 15 1 Condition 2 111.3 153.2 96.0 128.4  1.55 <1.0 320 ◯ ◯ example 16 1 Condition 3 180.3 140.3 100.9 140.5  1.22 <1.0 420 ◯ ◯ example

TABLE 5 Test results Cr/Fe concentration Pitting ratio in passive film Surface potential Clad- Aver- portion and parent layer σ measurement CPT test ding age phase portion phase 70° C. evaluation(A Surface mate- Production Gs Gs Gs Gs (relative value area ratio (V′C100 mV STM G48-03 treatment No. rial condition (60)_(L) (60)_(D) (60)_(C) (60) based on AES (%) vs SCE) Method E) cost Remarks 18 1 Condition 5 154.6 100.9 1000.0 339.1 0.92 12.6 77 X X Comparative example 19 1 Condition 1 129.3 115.6 100.6 115.3 1.51  3.4 220 X ◯ Comparative example 20 1 Condition 2 150.0 112.3 100.8 118.9 2.11 <1.0 342 ◯ ◯ example 21 1 Condition 3 135.3 136.5 150.3 139.7 1.89 <1.0 500 ◯ ◯ example 22 1 Condition 4 115.6 113.6 115.9 114.7 1.44  8.8 129 X ◯ Comparative example 23 1 Condition 5 123.6 106.6 109.6 111.6 2.50 14.3 99 X Δ Comparative example 24 2 Condition 1 100.3 98.6 126.0 105.9 1.65  5.5 130 X ◯ Comparative example 25 2 Condition 2 145.3 103.6 125.3 119.5 1.66  2.1 255 X ◯ Comparative example 26 2 Condition 3 136.3 126.3 140.6 132.4 2.31 <1.0 390 ◯ Δ example 27 2 Condition 4 121.3 123.6 100.9 117.4 1.89 12.6 65 X ◯ Comparative example 28 2 Condition 5 116.3 123.3 115.6 119.6 1.55 19.3 28 X ◯ Comparative example Underlined values are Comparative examples.

In this regard, methods of measuring the Cr/Fe concentration ratio, the glossiness, and the pitting potential, a CCT test method, and a corrosion resistance evaluation method are as described below.

Cr/Fe Concentration Ratio

The atomic ratio (atomic percent) of each element (Fe, Cr) was determined from each element profile measured by using AES (Auger Electron Spectroscopy) (name of apparatus: PHI MODEL 660 produced by PHISICAL ELECTONICS accelerating voltage: 5 kV amount of current of sample: 0.2 μA measurement region: 5 μm×5 μm) while sputtering was performed in the depth direction. Table 6 shows the numerical values of measurement results.

Table 6 shows an example of Cr/Fe distribution in the depth direction. Cr/Fe ratio in a passivation file varies by a circumstance and a corrosion resistance is improved by a concentration of Cr in the passivation film. A is defined Cr/Fe ratio in a parent stainless steel and B is defined Cr/Fe ratio in a passivation file. Values of B/A are listed in a column of Ratio of Cr/Fe relative to parent phase. The higher B/A, the more excellent corrosion resistance can be obtained for increased Cr concentration.

FIG. 1 shows the relationship between the measurement time and the values of Cr and Fe. In Table 6, “Ratio of Cr/Fe relative to parent phase” refers to a ratio of Cr/Fe measured with respect to time of sputtering from the surface layer to the Cr/Fe atomic ratio (here 0.33) in the parent phase. In this connection, as shown in FIG. 1, a region in which the values of Cr and Fe were almost constant values was specified to be a parent phase portion, and a region with a sputtering time shorter than that was defined as a passivation film portion. In the passivation film portion, the value at the site exhibiting a highest Cr/Fe value was defined as the Cr/Fe concentration and was compared with the Cr/Fe concentration in the parent phase portion.

TABLE 6 Sputtering Ratio of Cr/Fe relative time (min) Cr Fe Cr/Fe to parent phase 0 2 5 0.40 1.21 1 6 13 0.48 1.44 2 8 19 0.45 1.35 3 10 26 0.38 1.14 4 11 31 0.35 1.06 5 12 36 0.35 1.05 6 13 39 0.33 1.01 7 14 44 0.32 0.97 8 15 45 0.33 1.00 9 15 48 0.31 0.94 10 16 50 0.33 0.99 11 17 53 0.33 0.99 12 17 55 0.32 0.97 13 18 57 0.32 0.97 14 18 57 0.32 0.96 15 19 59 0.32 0.98 16 20 59 0.33 1.01 17 20 61 0.32 0.97 18 20 62 0.32 0.97 19 20 63 0.32 0.97 20 21 62 0.33 1.00 21 21 63 0.34 1.02 22 21 64 0.32 0.97 23 21 65 0.32 0.98 24 21 64 0.33 1.01 25 21 64 0.32 0.98 26 20 63 0.32 0.98 27 21 64 0.33 1.01 28 21 63 0.33 1.01 29 21 64 0.33 1.00 30 21 65 0.33 1.00

Glossiness

The surface glossiness was measured in conformity with JIS Z 8741 (1997) “Specular glossiness-methods of measurement” by using a multi-angle gloss meter GS series GS-1K produced by Suga Test Instruments Co., Ltd., at an angle of 60 degrees. Measurements were performed in three directions of the rolling direction (L), the perpendicular direction (C), and the direction at an angle of 45 degrees with respect to the rolling direction (D), the average glossiness indicator was determined on the basis of Formula (2) described below, and the resulting average glossiness indicator was specified to be the anisotropy of the surface glossiness.

average Gs(60)=(Gs(60)_(L)+2×Gs(60)_(D)+Gs(60)_(C))/4   (2)

In this regard, average Gs(60) represents average glossiness indicator, Gs(60)_(L) represents glossiness in the rolling direction (L), Gs(60)_(C) represents glossiness in the perpendicular direction (C), and Gs(60)_(D) represents glossiness in the direction at an angle of 45 degrees with respect to the rolling direction (D).

Pitting Potential (V′c100)

As for the indicator of the corrosion resistance in a salt damage environment, a pitting potential measurement was performed, where the measurement temperature was specified to be 70° C. and other measurement conditions were in conformity with JIS G 0577. The potential when the current density reached 100 μA/cm² was specified to be the pitting potential and was expressed in V′c100 (mV vs. SCE). The case where this pitting potential was 300 mV or more, the corrosion resistance was evaluated as good.

CPT (Critical Pitting Temperature)

To evaluate the pitting resistance, a sample 30 mm w by 50 mm l was subjected to the CPT test in conformity with ASTM G48-03 Method E, and the corrosion resistance was evaluated. Here, CPT refers to a critical pitting corrosion occurrence temperature and the threshold value of evaluation was specified to be 40° C.

That is, a symbol ‘◯’ indicates CPT of 40° C. or higher (acceptable) and a symbol ‘X’ indicates CPT of lower than 40° C. (unacceptable).

Area Ratio of Amount of Precipitation of σ Phase of Surface

Structure of the surface was observed and the σ phase area ratio was determined by image processing. In this regard, a sample subjected to 40% NaOH electrolytic etching was used for the structure observation.

As is clear from Tables 4 and 5, in our examples, stainless clad steels which have high pitting potentials, which are excellent in CPT evaluation, and which are excellent in rusting resistance are obtained.

Meanwhile, in the Comparative examples, the pitting potential is low or at least one of CPT evaluation is inferior. 

1-3. (canceled)
 4. A stainless clad steel comprising a stainless steel having Pitting Index represented by Formula (1) of 35 or more as a cladding material, wherein a ratio of Cr concentration (atomic percent)/Fe concentration (atomic percent) in a passivation film portion of the cladding material to Cr concentration (atomic percent)/Fe concentration (atomic percent) in a parent phase portion of the cladding material is 1.20 or more and, in addition, an amount of precipitation of a (sigma) phase of a surface of the cladding material is 2.0% or less on an area ratio basis, Pitting Index=(Cr+3.3Mo+16N)   (1) where Cr, Mo, and N indicate contents (percent by mass) of their respective elements, and no inclusion is specified to be
 0. 5. The stainless clad steel according to claim 4, when the average glossiness indicator Gs(60) calculated by Formula (2) of the cladding material is 60 or more, where the specular glossiness at a measurement angle of 60° defined in JIS Z 8741 is measured in a rolling direction (Gs(60)_(L)), in a direction perpendicular to the rolling direction (Gs(60)_(C)), and in a direction at an angle of 45 degrees with respect to the rolling direction (Gs(60)_(D)), Gs(60)=(Gs(60)_(L)+2×Gs(60)_(D)+Gs(60)_(C))/4   (2).
 6. The stainless clad steel according to claim 4, wherein the average glossiness indicators Gs(60)_(L), Gs(60)_(C), Gs(60)_(D) and Gs(60) are 60 or more, where the specular glossiness is at a measurement angle of 60° as defined in JIS Z
 8741. 7. The stainless clad steel according to claim 5, wherein the average glossiness indicators Gs(60)_(L), Gs(60)_(C), Gs(60)_(D) and Gs(60) are 60 or more, where the specular glossiness is at a measurement angle of 60° as defined in JIS Z
 8741. 